High-frequency electrostatic heating of plastics



1943 6 sheets-sheet I H. F. MaCMlLLIN Filed March lO,

HIGH FREQUENCY ELECTROSTATIC HEATING oF PLASTICS 6 Sheets-Sheet 2 H. F. MaCMILLlN Filed March lO, 1943 HIGH FREQUENCY ELCTROSTATIC HEATING OF PLASTICS m L RW Y z mm me am SX a.. o.. ww www Inn A fl) n.. www j F {JL} ..5 m l d ow euh- .2. S 2x .N n n n A* :39 u e w o. m

`N m .Nm MN o.

Oct. 16, 1945.

Oct. 16, 1945.

H F. MacMlLLlN 2,386,966

HIGH FREQUENCY ELECTROSTATIC HEATING OF PLASTICS Filed March 10, 1943 6 Sheets-Sheet 3 ATTGRNEV @en 16, 1945. H. F. Mach/umm 2,386,966

HIGH FREQUENCY ELECTROSTATIC HEATING OF PLASTICS Filed March-10, 1943 6 Sheets-Sheet 4 NVENTOR Oct. 16, 1945. H. F. MacMlLUN 2,386,966

HIGH FREQUENCY ELECTROSTATIC HEATING OF PLASTICS Filed March l0, 1943 6 Sheets-Sheet 5 Il .Q LS, CJ A C:

TH L52 HEAT 36 o 0 lOZI'EIJI'U" o \05 Il :39 n

Q I I I V /ff/// Y@ /j INVENTOR HOWARD F. MncMwuN /////////////J fi; msNrM Oct. 16, 1945. 2,386,966

HIGH FREQUENCY ELECTROSTATIC HEATING OF PLASTICS l-l4 F. MHCMILLIN Filed March 1.0, 1943 6 Sheets-Sheet 6 INVENTOR HOWARD EMM M/LL/N,

ATTORNEYS mow am om m@ Inma Q www uw SN /7/,7 //////7 S2 N Y B f m2 NGN /J/l ///////V l/Mw/a//f I/ Q l// n Q @Q3 N@ m2 ,v d .mllw 2; t s: H 11111111 l I I i 1 v./////////,7//// Mh.||||||l Il n um ---,luwa -|-l-|- Tlrlllllllomm IT NS v///V n2 mi l| rlllmlel LL s I `m` Q2 E m2 `2 .n QQ n2 -when the heating effect of the Patented Oct. 16, 1945 HIGH-FREQUENCY ELECTROSTATIC HEATING OF PLASTICS Howard F. MacMillin, Mount Gilead, Ohio, as-

signor to The Hydraulic Development Corp., Inc., Wilmington, Del., a corporation of Dela- Ware Application March 10, 1943, Serial No. 478,677

22 Claims.

This invention relates to a method and apparatus for injection molding of plastic material.

An object of the invention is to provide a method and apparatus for injection molding of plastic material wherein the 'plastic material is heated to plasticity by means of a high frequency electric current and to intensify the heating effect upon the plastic material by means of a high frequency Acurrent just before the material is ejected from the injection machine.

Another object of the invention is to provide a method and apparatus for heating plastic material that is to be ejected from an injection machine using high frequency current and arranging the apparatus so that there is no metal within the high frequency field.' 1

Another object of the invention is to provide a method and apparatus for heating plastic material by the use of high frequency current so that only the plastic material is heated when passing through the injection cylinder of the machine whereby there will be no residual heat tending to elevate the temperature of plastic material j high frequency current is stopped.

Another object of the invention is to provide a method and apparatus for heating of plastic material within an injection cylinder by means of high frequency current so that the plastic material is elevated'to a temperature somewhat below the temperature at which it is to be ejected into the'mold and the intensity of the heating' effect is increased in the area adjacent the nozzle orifice of the injection machine so that the temperature of the material is elevated just before the material enters the nozzle to increase the plasticity of the material for injection into the mold.

Another object of the invention is to provide a method and apparatus for heating a thermosetting resin by means of high frequency current while within an injection cylinder so that the thermosetting resin is initially raised to a temperature to plasticize the same but to hold the temperature of the material below the critical temperature and to provide an intensification of the heating effect upon the thermosetting resin just before it passes into the injection nozzle of the injection cylinder so that it is elevated to or slightly above the critical temperature to begin polymerization of the material at the time it is injected into the mold.

Another object of the invention is to provide a method and apparatus for heating plastic material within an injection cylinder by means of high frequency heat in a manner that the high terial within an injection cylinder by means of a high frequency current ina manner that only the plastic material is elevated in temperaturev and the injection cylinder of the machine can be cooled to prevent the same from retaining heat absorbed from the plastic material and thereby prevent any residual heat in the injection cylinder from effecting the temperature of the plastic material.

Another object of the invention is to provide an apparatus for injection molding of plastic material wherein the injection cylinder of the injection machine provides one electrode from a source of high frequency current and the second electrode is carried by the injection plunger in a manner that it is adapted to precede the-injection plunger in its movement through the injection cylinder so that the plastic material within the injection cylinder is heated within the high frequency field set up between the electrode carw ried by the plunger and the injection cylinder.

It is another object of the invention to pro@ vide an apparatus constructed in accordance with the foregoing object wherein the electrode carried bythe injection plunger is positioned at a first location whereby the high frequency heating effect on the plastic material is carried out uniformly throughout the mass of plastic ma terial in the injection cylinder and the electrode carried by the plunger can then be positioned at a'second location so that there will be an inm tensied field of high frequency current set up through which the plastic material must pass before entering the injection nozzle of the ma. chine so as to increase the plasticity of the material just prior to the entry of the same into the nozzle.

It is another object of the invention to provide a system of control for the injection machine for automatically carrying out the operations here-n tofore set forth in the foregoing objects wherein the injection plunger and the Aelectrode carried thereby are separated independently from one another.

Further objects and advantages will become apparent from the drawings and the following description.

IIn the drawings:

Figure 1 is a cross sectional schematic view of a machine incorporating features of this invention.

Figure 2 is a schematic view of a machine involving features of this invention, as shown in Figure 1, and the hydraulic circuit for operating the same. y

Figure 3 is an electricaly diagram of the controls for operating the machine disclosed in Figure 1.

Figures 4, 5, 6 and 7 are diagrammatic views representing the position of the injection plunger and the central electrode therein during various periods of the cycle of operation of the machine.

Figure 8 is a schematic cross sectional view of a slightly modified arrangement of the machine of this invention.

Figure 9 is a schematic wiring circuit for automatically controlling the machine disclosed in Figure 8.

Figures 10, 11 and 12 are diagrammatic views showing the elements of the machine in various positions during the cycle of operation.

Figure 13 is a cross sectional view, somewhat schematic, showing high frequency heating applied to an injection machine.

Figure 14 is a cross sectional view of an extrusion machine, somewhat schematic, showing high frequency heating applied thereto.

In this invention the injection machine consists of -an injection cylinder I having an injection chamber II. An injection nozzle I2 is secured to the front end of the injection cylinder I0 by means of the screw thread portion I3. The injection chamber II is provided with a uniform diameter throughout the length thereof, the front wall I4 thereof being tapered to form a cone-shaped section. This cone-shaped section can either be provided in the front wall of the chamber II or, as illustrated, can be placed in the nozzle I2. The nozzle I2 has an orifice I that tapers outwardly from the injection chamber II so that the sprue can be drawn from the nozzle orice I5 when the molded piece is removed from the mold cooperating with the nozzle I2. A cooling chamber I6 may be provided in the injection cylinder I0 for encircling the portion of the cylinder in which the plastic ymaterial is to be heated, the purpose of this cooling chamber I6 being hereinafter explained. Suitable fluid conducting conduits I'I and I8 are provided for circulating cooling fluid through the chamber I6, a valve I9 being provided to control the iiow of iiuid through the conduits I'I and I8.

The injection cylinder I0 has a feed hopper associated therewith into which the granular or powdered plastic material is placed for feeding to the injection cylinder II. The hopper 26 is provided with an angularly disposed feed chute 2| in which there is placed a feed screw 22 that is driven by an electric motor 23. The feed chute 2I communicates with a. feed opening 24 provided in the injection cylinder I0 that conveys granular or powdered plastic material into the injection Achamber II. The hopper 20 and the associated feeding mechanism is suitably supported upon or above the injection cylinder I0 in any conventional manner.

A movable die is positioned adjacent the injection nozzle I2 and is adapted to engage the Same in a manner to be hereinafter described.

A second movable die 26 co-operates therewith for forming a suitable mold cavity. The movable mold 26 is actuated by means of a hydraulic motor ,21 for opening and closing the molds. The motor 21 is of the double acting type whereby the mold 26 will be moved under power in either direction, the motor having fluid conducting conduits 28 and 29 for conveying uid to and from the motor 21. The dies 25 and26 are provided with suitable cored passages through which iiuid can be circulated for heating or cooling purposes. A fluid inlet connection 30 conducts fluid to the mold or die 26 as controlled by the valve 3l and a conduit 32 conducts i'luid from the mold 25, the conduit 33 interconnecting the molds 25 and 26 whereby the heating or cooling fluid can circulate through these molds.

When the machine of this invention is used for injection molding of thermoplastic resins the molds 25 and 26 maybe maintained sufliciently cool merely by the normal circulation of room air around the molds or a cooling iiuid can be circulated through the'same for setting the thermoplastic material. However, when the machine is to be used for injection molding of thermosetting resins then a heating nuid is circulated through the molds 25 and 26 to elevate the temperature of the thermosetting material above the critical temperature for setting the same.

An injection plunger generally designated by the numeral 35 is adapted to be reciprocated within the injection chamber II by means of a hydraulic motor 36 that is of the double acting type for moving the injection plunger 35 under power in either direction of movement. Fluid conducting conduits 31 and 38 are provided'for conveying fluid to and from the hydraulic motor 36 on opposite sides of the piston 39.

The injection plunger 35 consists of a metal sleeve member 40 that slides within the injection chamber II and is finished to close tolerances with the injection chamber II to providea seal against passage of plastic material between the plunger 35 and the injection cylinder I0. The sleeve 40 carries an electrical insulating sleeve 4I that is constructed of a hard fibrous ished to close tolerances with the electrode 42 to prevent passage of plastic material between the electrode 42 and 'the sleeve 43.

The front wall 46 of the injection plunger 35 is provided with the same configuration as the end wall I4 of the injection chamber II so that the end wall 46 of the injection plunger 35 can abut the wall I4 of the injection chamber II with no clearance provided therebetween. The front wall 4l of the electrode 42 is also provided with a configuration that conforms with the configuration of the wall I 4 of the injection chamber I I, so that when the electrode 62 is moved forward within the injection chamber II that the wall 4l of the electrode will be spaced equidistant from the wall I4 by the same amount that the periphery of the electrode 42 is spaced radially from the inner periphery of the injection chamber II for reasons which will be hereinafter disclosed.

As previously referred to, the electrode 42 is movable axially within the injection plunger 35. To accomplish this operation the rear end of the electrode 42 is provided with an electrical interial within the injection chamber il.

sulated bushing 48 that can be constructed of the same material as the sleeve 4| of the injection plunger 35. The electrode 42 is suitably secured to the bushing 48 such as by means of the nut 49 and is engaged by an electric cable 50 that conducts electric current to the electrode 42.

The bushing 48 is secured to a yoke 5| that extends through a slot 52 provided in the injection plunger 35. Opposite ends of the yoke 5| are connected to the plungers 53 and 54 extending from the hydraulic motors 55 and 56, respectively. the plunger 53 and 54 extending from the pistons 51 and 58 of the motors 55 and 56, respectively. The hydraulic motors 55 and 56 provide means for reciprocating the electrode 42 within the injection plunger 35 independently of the reciprocation of the injection plunger 35 by means of the hydraulic motor 36 in a manner that will be hereinafter described. Fluid conducting means 59 and 60 are provided on opposite ends of the hydraulic motor 55 for supplying fluid to and from the motor 55 on opposite sides of the piston 51. Similar iiuid conducting means 62 and 63 are provided for the hydraulic motor 56 for conveying iiuid to and from the motor on opposite sides of the piston 58.

The electricv cable 50 that is secured to the electrode 42 is connected to one terminal 64 of a source of high frequency electric current 65. The second terminal 66 of the high frequency source 65 is connected to the frame of the injection machine, and as illustrated in Figure 1 is connected to the injection cylinder I0, this cylinder representing a part of the frame of the Source of heat The plastic material in the injection cylinder I is heated by the use of high frequency electrostatic heating. The principle of operation of high frequency electrostatic heating should not be confused with high frequency induction heating. In high frequency induction heating, the high frequency magnetic field creates eddy currents in the metal that is being heated. The eddy currents are dissipated and converted into heat at or near the surface of the metal. High frequency electrostatic heating is the result of internal heating of a material rather than a heating of the surface of the material for conduction through the mass. One explanation of the effect of the high frequency electrostatic heating is that the molecules of the material through which the high frequency is carried are repeatedly changed in shape on applying the electric field to the molecules. The frequency of the current that is applied to the mass determines the number of times the molecule is deformed per second and in the case of high frequency electrostatic heating this frequency may be from 1,500,000 to 10,000,000 cycles.

Figure shows the electrode 42 and the injection plunger 35 in their respective positions during the heating cycle for heating the plastic ma- As previously described the source of high frequency 65 has the terminals 64 and 66 thereof connected to the electrode 42 and the injection cylinder l0, respectively. When current is applied to the terminals 64 and 66 of the source of high frequency 65, there is a high frequency field set un between the electrode 42 and the injection cylinder i0 whereby the plastic material between the electrode and the injection cylinder is heated due to the molecular friction heretofore referred to. This electrostatic heating of the plastic material within the injection chamber Il has the advantage of heating the entire mass uniformly throughout its entire cross section so that there will be no localized heating effect upon the plastic material as normally occurs when heating plastic material by surface engagement only. Such high frequency electrostatic heating is particularly advantageous for heating materials that have low heat conducting factors because the high frequency current works uniformly throughout the entire mass of the material. Such high frequency currents as desired to be used in this process can readily be obtained by a conversion of direct current into radio frequency current of the desired frequency by means of mercury vapor amplifier tubes in any conventional manner. Therefore, when the use of high frequency current is referred to in this specification it is to be considered in terms of high frequency electrostatic heating as distinguished from high frequency induction heating.

One of the principal factors in controlling the heating effect upon materials that are heated by high frequency electrostatic heating is to maintain the electrodes equidistantly spaced from one another so that the area between the electrodes is substantially constant whereby uniform heating of the mass between the electrodes is obtained.

As theelectrodes are brought closer together in certain portions, or sections thereof, the heating effect upon the material between the electrodes is intensified because the electrostatic held is intensified between the electrodes. This characteristie of intensification of heating effect, when the electrodes are close together in certain areas, is used to advantage in the present invention to intensify the heating effect upon the plastic material before it is ejected through the injection nozzle of the injection machine in a manner that will be hereinafter described.

Hydraulic controls The hydraulic system ofcontrols for the machine consists of a pump 10 that is adapted to produce a relatively constant pressure and may be of the variable delivery type having suitable controls associated with the pump for shifting the same to neutral upon attainment of a predetermined pressure, or could be a constant delivery pump having relief valve means associated therewith for bypassing excess fluid delivered from the pump when the pressure on the delivery side reaches ya predetermined maximum. As shown in Figure 2 a control 'Il is connected by means of a conduit 'l2 with a discharge conduit 13 from the pump 'l0 to regulate the pressure in the conduit 73. The pump l0 receives fluid from the reservoir 14 through a conduit 15.

The fluid conducting means 3l and 38 of hydraulic motor 36 for operating the injection plunger 35 are connected to a self-centering closed center four-way valve 76. rI'his valve 1S is connected to the pump 10 by means cf a conduit 'l1 and with the supply reservoir through a conduit 78. Solenoids 78 and 80 engage the Valve member 8l positioned within the valve casing 82 of the valve 16 to shift the member to left or right according to energization of either of' the solenoids. When the valve member 8i of the valve 'i6 is in the center position there will be no ow of fluid to either end of the hydraulic motor 36 thus to prevent movement in either direction. When either solenoid 19 or 80 is energized iiuid will flow through one of the fluid conducting means 31 or 36l for actuating the hydraulic motor 36. The spring members 83 and 64 center the valve member BI in the valve 16 when the solenoids are both de-energized.

A similar self-centering closed center 4-way valve 85, having actuating solenoids 89 and 90, is connected to the fluid conducting means 59 and 60 of the hydraulic motor 55 and the fluid conducting means 62 and 63 of the hydraulic motor 66 are connected in parallel with the fluid conducting means 59 and 60 so that the one valve 85 When the movable die 26 is moved out of engagement with the movable die 25 the hydraulic motors 9| and 92 will cause the die 25 to move away v from the nozzle I2 of the injection cylinder I0. This movement breaks the thermal connection between thedie 25 and thenozzle I2 so that when thermosetting material is being injection molded in the machine and heat is being applied to the dl'es 25 and 26 there will be no transfer of heat into the plastic .material in the injection cylinder II through the nozzle I2. Also, this movement of the die 25 relative to the nozzle I2 causes the sprue in the nozzle I2 to be drawn therefrom, the

tapered cross section of the nozzle orifice I5 permitting this withdrawal of the sprue from the nozzle. Y

Operation Assuming that the injection pmnger as and the electrode 42 have just been retracted to their l rearmost position, an electric control will deenergize the valves 16 and 85 whereby they will return to closed center position'so that no fluid will flow to the hydraulic motors 55, 56 and 36,

y thereby stopping the injection plunger and the electrode 42 in their rearmost position. Simultaneously, the electric motor 23 will rotate the feed screw 22for moving a granular plastic material into the injection chamber II of the injection 'cylinder I0. The position of the elements of the apparatus at this time is shown in Figure 1.

As soon as a suitable timing control has timed the feeding of plastic material into the injection chamber II to feed a predetermined quantity therein, an electric control will be actuated to energize the valves 16 and 65 whereby fluid will be supplied to the left-hand side of the pistons 'in the hydraulic motors 55, 56 and 36, thereby causing the injection plunger 35 and the electrode 42 to move forward into the injection chamber I-I. The areas of the pistons in the hydraulic motors and 56 are proportioned to the area of the end surface 41 of the electrode 42 relative to the area of the piston in the hydraulic motor 36 to the area of the end 46 of the plunger 35 so that the same fluid pressure applied to the motors 55 and 56 will cause the electrode 42 to move forward into the injection chamber II at a greater gte than the movement of the injection plunger The injection plunger 35 and the electrode 42 will thus move forward through the injection chamber II simultaneously so that the injection plunger 35 will carry the granular plastic material forward into the forward end of the injection chamber il and the electrode 42 will extend from the forward end thereof outwardly from' the injection plunger 35. This action will continue until the electrode 42 actuates an electrical control that de-en'ergizes the valve 85 to permit the same to center itselfand thus stop the movement of the electrode 42. This position of the electrodeis illustrated in Figure 4 wherein the front wall 41 of the electrode v42 is spaced from the front wall I4 ofthe injection chamber II a distance that is equal to the radial distance between the outer periphery of the electrode 42 and the inner periphery of the injection chamber II.

The injection plunger 35 has not stopped its forward movement during this time but continues its forward movement to compact the granular plastic material within the forward end of the injection chamber II. When the granular plastic material has been compressedto a predetermined degree the pressure required to move the piston 39 of the hydraulic motor 36 increases so that this pressure will reflect a predetermined compactness of the granular material. This pressure rise in the'hydraulic motor 36 will actuate aA pressure control for cle-energizing the valve 16 to permit the same to center itself and thus stop further forward movement of the injection plunger 35.

The position of the electrode 42.and the injecj tion plunger 35 with the plastic material precompressed inthe forward end of the injection chamber is shown in Figure 5. As soon as the injection plunger 35 has reached the position, as shown in Figure 5, an electric control will be operated to cut on the current to the electrode 42 and the injection cylinder I0 thereby setting up a highfrequency field between these two members. As shown in Figure 5, and as previously stated, the surface of the electrode 42 is.

spaced equidistantly from the surface of the injection chamber II so that the electrostatic field set up' between the electrode 42 and the injection cylinder I0 will be substantially uniform throughout the entire mass of plastic material within the injection cylinder II to uniformly heat this mass. While'the heating step has just been re- 'ferred to as being initiated when the injection plunger 35 arrives at the position shown in Figure 5, it is entirely within the purview of this invention that the heating cycle can start as soon as the electrode 42 has been positioned as shown in Figure 4, that is at the time it is spaced from the front wall of the injection chamber II, regardless of the position of the injection plunger 35. These two operations can therefore overlap to a certain extent. v

Considering the heating of th'e plastic in the injection chamber II is started when the injection plunger 35 stops in the position shown in Figure 5, a timing control is actuated at the time the injection plunger 35 stops to time the heating cycle, in order to bring the temperature of the granular plastic material in the injection chamber II up to plasticity.

After a predetermined time period has elapsed, as controlled by the timing control, the valve will be energized to again permit fiuid to enter through the conduits 59 and 62 of the hydraulic motors 55 and 56 td again move the electrode 42 `temperature that it can be to a position that is closely adjacent the front wall I4 of the injection chamber II, as shownin Figure 6. It will be apparent from the position of the electrode that the electrostatic field between the end wall 41 of the electrode 42 and the wall I4 of the injection chamber II will be greatly intensified so that plastic material when passing through the small space provided between the end of the electrode 42 and the wall I4 of the injection chamber II will be elevated in temperature considerably above the temperature of the general body of plastic material within the forward end of the injection chamber II. This intensification of heat at this time permits the plastic material to be raised to a sufficient high readily injected into the mold. It is to be assumed, of course, that prior to this time that the molds 25 and 26 have been closed by the hydraulic motor 21 by previous actuation of the valve B6 by suitable controls that Were actuated by either the injection plunger 35 or the electrode 42 during a previous stroke of movement.

As soon as the electrode 42 arrives at its forward position as shown in Figure 6, an electric control will be actuated to again energize the valve 16 to permit fluid te owthrough the conduit 3T into the hydraulic motor 36 and thus advance the injection plunger 35 to produce an injection operation. The plastic material that is held at a temperature of plasticity in the injection chamber II has a substantial elevation of temperature imparted thereto when passing between the end of the electrode 42 and the wall I4 as heretofore described.

The injection plunger 35 continues its forward movement until substantially all of the material is discharged from the injection chamber II as shown in Figure 7. When the cavity in the dies 25 and 2G is filled with plastic material the injection plunger 35 can no longer move in a forward direction so that the pressure in the hydraulic motor 36 will increase rapidly at this time.

This increase of pressure in-the hydraulic motor 36 causes actuation of a control member that cuts off the electric energy from the electrode 42 and the injection cylinder I so that the plastic material will not be overheated or burned at the sprue opening. This control for cutting olf the heat is responsive to the first increase in pressure developed in the fluid motor 36 indicating that the mold has been filled. There is usually a slight movement of the injection plunger after this time until a maximum pressure is reached as controlled by the maximum pressure setting of the pump 10 so as to insure complete filling of the mold and forcing the plastic material into the remote corners of the die cavity. Therefore, this final movement of material through the sprue opening will be at a time when the heat has been removed and the last material entering the sprue opening will be at a somewhat lower temperature than the material that has heat imparted thereto during the intensification period.

After a predetermined dwell period as determined by the timing control the valves 16 and 85 will be operated to cause fluid to be conducted to the right-hand end of the hydraulic motors 55, 53 and 36 to thereby produce retraction of the injection plunger 35 and the electrode 42. Simultaneously, the valve 86 is actuated to cause the motor 21 to open the molds 25 and 26. The machine is then ready for a new cycle of operation.

When using thermoplastic resins in the machine hertofore described and operating the same on the cycle of operation as set forth herein, the plastic will first be heated to a temperature somewhat below the temperature at which it is to be injected into the mold and while it is being injected will have the temperature thereof raised substantially above the previous level. In the case of thermosetting resins, when being injection molded in the machine, this intensication of heating of the plastic material just prior to its ejection from the injection chamber serves the purpose of raising the temperature of the material above the criticall point so that it will immediately begin polymerization upon entering the mold. The first heating of the material is such that the temperature thereof is not raised to the critical point. Also, since the plastic material itself is the only substance being heated there will be no temperature lag in the elements of the machine causing the temperature of the Y plastic material in the injection chamber to raise to polymerization temperature. In fact, since the electric current is turned off to stop the heating effect before the injection plunger has actually stopped movement, the small amount of plastic material between the end of the electrode 42 and the wall I4 of the injection chamber I I will be forced into the sprue passage I5 of thenozzle I2 so that there Will be no high temperature material remaining in the injection chamber II around the nozzle orifice I5 when the injecticn plunger 35 has completed the injection stroke. Therefore, any material that polymerizes will be that in the mold land in the sprue so that when the mold 25 is moved relatively to the nozzle I2 that the sprue will be pulled from the nozzle to clear the same of all polymer-ized plastic material. i

The cooling chamber I6 previously referred to is provided to prevent the temperature of the injection cylinder l0 from elevating and thus producing a heat lag in the plastic material within the injection chamber II. This cooling of the injection cylinder i0 will have special advantage with connection to the thermosetting resins and a more accurate control of the heating of the Amaterial can be obtained because there will be Electric circuit An electric circuit showing one manner in which the machine heretofore disclosed can be operated .automatically is disclosed in Figure 3. When the injection plunger 35 reaches its rearmost position it strikes limit switch LS to energize contacter A whereby contacts A1 and A2 are closed to energize a timer TA for controlling the feed cycle and the feed motor 23. When the timer TA has run its period of time the contacts TA1 will be closed to energize the contacter F which opens the normally closed contacts F2 and thereI by deenergize contacter A to open contacts A1 and A2 and permit contacts A3 and A* to close. Opening of contacts A1 and A2 permits the timer TA to reset and stops 'the feed motor 23.

Closing of the contacts A3 and A4 energizes solenoid 8D of the valve 'i6 and the solenoid 9G of the valve to cause the injection plunger and the electrode 42 to move forward in a manner heretofore described at the different rates ci speed. When the electrode 42 reaches the posi tion shown in Figure 4, a limit switch LSl is opened thereby de-energizing the solenoid 80|to stop forward movement of the electrode 42, the injection plunger continuing its forward movement until the pressure operated control T1 is operated due to the increasing pressure produced in the motor 38 upon compression of the plastic material in the injection cylinder. Closing of the pressure control T1 energizes the contactor B closing the contacts B2 and B3 whereby electric current is supplied to the electrode 42 and the injection cylinder I to heat the plastic material and at the same time a second timer 'I'B is started in operation. When the timer TB has run its cycle the contacts 'lCBl ,are closed to again energize the solenoid 90 of the valve 85 to advance the electrode 42 to the position shown in Figure 6 at which time a limit switch LS is actuated energizing the contactar lCz that closes contact Cl to hold the contactar in and opens Acontact C* to de-energize the solenoid 90 and thus stop forward movement of the electrode 42. Simultaneously, contacts C3 are closed to energize the solenoid 80 of the valve 18 and again move the injection plunger 35 in a forward motion ige ejlelct plastic material from the injection chamr The injection plunger 35 continues its forward motion until the molds are filled and the pressure begins to rise in the hydraulic motor 36 whereby the pressure operated control I2 is closed .to energize the contactor D which closes the vcontacts D1 and D2 for holding the contactor in and for energizing a timing control TC that regulates the dwell period while the plastic material is setting. At this time contacts D3 are opened thereby de-energizing contacter B and opening contacts B3 to cut oil the electric current from the source of high frequency and also de-energize the heating timer control TB so that it can reset.

When the timing control TC runs its period it will close contacts TC1 thereby energizing a contactor E to close contacts El and Ez and thereby" energize the solenoid 19 of the valve 16, the solenoid 89 of the valve 85 and the solenoid 88 of the valve 86 to reverse the injection plunger 35, the electrode 42 and retract the mold member 26 whereby the machine is in condition for another cycle of operation.

The mold member 26 can be closed upon the moldv member 25 at any convenient time in the previous cycle of operation before the injection plunger moves on the injection stroke.

mold member 26 is closed upon the mold member 25 when the contactor B is energized closing the contacts B4 to energize the contacter G which in turn closes the contacts (Fr1 and G2 and thereby energize the solenoid 81 of the valve 86 to advance the mold 26.

M odification, machine Therefore, in the electric circuit shown herein, the

The injection plunger |08 is connected to a yoke |06 that is operated by means of a hydraulic motor |01, a motor being positioned on each side of the yoke |06 to actuate the same.

An electrode |08 is slidably mounted axially within the injection plunger |06 and extends from the rear end thereof into engagement with a piston |09 provided within al cylinder |I0 that forms a hydraulic motor for reciprocating the electrode 08 within the injection plunger |05. Thus far it will be apparent that the arrangement of the machine is substantially the same as that heretofore described in Figure l except that the parts have been slightly rearranged.

The electrode |08 has a cable ||2 connected thereto that terminates on one of the terminals of a source of high frequency, a second cable ||3 being connected to the injection cylinder |00 in the same manner as heretofore described so that a high frequency field will be established between the electrode |08 and the injection cylinder |00 for heating the plastic material within the injection cylinder |00 as fed therethrough by the injection plunger |05. iAn insulating member ||4 is positioned in the electrode |08 to electrically insulate the same from the machine frame.

The fluid motor for the electrode |08 has fluid conducting conduits ||5 and ||6 that are connected to a solenoid actuated 4-Way valve ||1 that is normally retained in one position for delivery of fluid through the line IIS by means of a spring I8 and when energized by a solenoid I9 will direct fluid through the conduit H5, thereby reciprocating the electrode |08 within the injection chamber |0i. The hydraulic motor |01 is provided with fluid conducting conduits |20 and |2| that are connected to a solenoid actuated 4- way valve |22 that is arranged to normally supply fiuid through the conduit |20, the valve being urged in one direction by means of the spring |23 and when urged in the opposite direction by means of the solenoid |24 will supply fluid through the conduit |2I, whereby the injection plunger |05 will be reciprocated in the injection chamber |0 I.

A source of pressure fluid is provided by means of the pump |25 that may be of either of the types heretofore referred to for maintaining pressure in the supply line |26 by means of the control element |21. The pump receives its fluid supply from a reservoir |28 through a conduit |29. The delivery from the pump |25 is conducted to the valve ||1 by a conduit |30 and to the valve |22 by means of a conduit |3i, the valves ||1 and |22 having return conduits |32 and |33, respectively. for returning uid to the reservoir |28. It is to be understood, of course. that the injection cylinder |00 cooperates with suitable molds |34 and |35 that can be heated, cooled and operated in the same manner as heretofore described in regard to the molds 25 and 26 in Figure 1.

Cycle of operationl Assuming the plunger |05 to be in its retracted" chamber the valve |22 will be actuated to operate the hydraulic motor |01 for advancing the plunger |05 through the injection chamber |0|. W n the injection plunger passes over the feed o ing |36 an electric switch is closed whereby high frequency current is supplied to the electrode |08 and, the injection cylinder |00 for establishing a high frequency field therebetween for heating theplastic material. During i this heating period the source of high frequency current is controlled by a temperature responsive device |31 such as a thermo-couple control to regulate the temperature of the plastic material it is in the position shown in Figure 1l so that there will be a continuous intensification of the heating effect upon the plastic material during the injection period.

When the cavities in the molds or dies |34 and |35 are filled, the injection plunger |05 is stopped from further forward movement due to the fact that the plastic material can no longer be ejected from the injection chamber |0| so that the pressure in the hydraulic motor operating the injection plunger |05.will rise. This rise in pressure will operate a pressure responsive control for actuating av timing control that regulates the dwell 'period of the plunger in its forward positlonto hold pressure on the plastic material in the molds during the time that the material is setting. This same pressure responsive control will also cause operation of the valve to actu- Yatelthe hydraulic motor and cause the elec- -trode |08 to be withdrawn to the position shown in Figure 12, which immediately stops the intensiiication of the heating effect upon the plastic the electrostatic heating within the plastic matev rial will be uniform throughout the entire mass.

The injection plunger |05 continues is forward stroke until the plastic material within the injection chamber |0| has been precompressed to a degree that the pressure required to move the injection plunger .forward increases in the hydraulic motor |01. At this time a pressure operated control is actuated to operate the valve whereby the hydraulic motor is allowed to move the electrode |00 forward within the injection chamber 0| to a position such as that shown in Figure 10, the injection plunger |05 being approximately in the position shown in Figure 10 with the plastic material in front of the same being held under precompression, it being understood that the injection plunger |05 has not stopped but this is one position of the injection plunger during one period of the injection cycle. While it is preferable to control the movement of the electrode |08 in its forward movement by the degree of precompresslon of the plastic material in theinjection chamber |0| yet this control could be by means of a limit switch that is actuated when the plunger I 05 reaches a predetermined position in its forward stroke.

The position of the electrode |08, as shown in Figure 10, is for the purpose of intensifying the heating effect upon the plastic 'material as it passes between the end of the electrode |08 and the end wall |39 of the injection chamber |0| in the manner as heretofore described.

With the injection plunger |05 still moving in a forward direction, plasticized material will now be ejected from the injection chamber 0| into the dies |34 and |35, the plastic material being raised in temperature above the plasticizins' DOint as held in the injection chamber i 0| when passing between the end of the'electrode |08 and the end wall |39 of the injection chamber. The injection plunger |05 continues its foi;

ward movement until the cavities in the molds material in the forward end of the injection chamber |0|. Also, the source of high frequency current is again placed under control of the temperature responsive control.

Since the pressure responsive control can be set to operate upon the first indication of a rise in pressure, which indicates that the mold is almost full, the electrode |08 can be withdrawn from the position shownin Figure 11 to the position shown in Figure 12 before the plunger |05 has actually stopped its forward movement. The final forward movement of the plunger to completely fill the mold, and to iill the voidthat is created by the withdrawal of the electrode |08 will move plastic material from the general mass of material in the forward end of the injection chamber |0| into the injection nozzle orifice |03 so that the material in the front end of the nozzle orifice will be material which is at a temperature below the temperature of the plastic material that has had the heat intensified therein before its passage into the nozzle. Therefore, plastic material at plasticizing temperature only will be in the forward end of the nozzle orifice |03 so that when the molds |34 and |35 are moved away from the nozzle |02 in a manner heretofore described with regard to the operation disclosed in lFigure 1, the sprue in the nozzle orliice will be withdrawn therefrom. As previously disclosed this operation of the electrode is particularly advantageous when using thermo-setting resins in the injection machine because the final material that is forced into the nozzle orifice |03 will be at a temperature below the critical temperature of the material so that it will not polymerize in the nozzle orifice, or at least there will be a por-u tion of the nozzle orifice that will contain material below polymerization temperature soA that the material which is polymerized in the nozzle orice can be withdrawn therefrom when the dies |34 and |35 are withdrawn from the nozzle 02.

When the timing control that regulates the dwell period has finished the time cycle thereof, Ithe valve |22 will be permitted to operate to again actuate the hydraulic motor |01 to withdraw the injection plunger |05 into the position shown in Figure 8, and the high frequency current will be cut ofi. to stop the heating effect thereof during the actual feeding cycle of fresh plastic material, at which time an electric control will be actuated to again start an automatic cycle of operation.

Electric circuit A schematic electric circuit showing one. manner in which the machine disclosed in Figure 8 can be operated consists of a limit switch LS1 that is engaged by the plunger |05 when in the completely retracted position as shown in Figure 8, thereby energizing the contacter A, the contacts B2 in this circuit being closed at this time as a result of a previous operation. The contactor A closes the contacts A1 and A2 thereby energizing the feed motor for the feeding apparatus and a timing control TA.

When the injection plunger is in its retracted position, limit switch LS2 has also been opened during the retraction movement of the plunger so that the high frequency current to the electrode |08 and the injection cylinder |00 is cut off to stop the heating of the plastic material when granular material is entering the injection chamber through the feed opening |36.

When the timing control TA has completed its cycle the contacts TA1 will be opened thereby deenergizing the contactor B, the contacts TB1 in this circuit having been closed by a previous operation in the cycle of operation. When the contacter B is de-energized, contacts B1 and B2 will be opened and contacts B3 will be closed to energizethe solenoid |24 of the valve |22 to thereby supply fluid through the conduit |20 of the hydraulic motor |01 .to move the injection plunger in a forward stroke.

When the injection plunger |05 passes over the I feed opening |36 the limit switch L52 will be permitted to close thereby cutting on the high frequency current to the electrode |08 to again start the heating of plastic material in the injection cylinder |0|.

The injection plunger |05 continues its forward motion 'to precompress the plastic material in thel injection chamber |0| until the pressure in the motor |01 increases to operate a pressure actuated control T1 which thereby energizes the contactor C for closing the contacts C1, C2 and C3. Contacts C2 energizes the solenoid ||9 to shift the valve |1 to supply fluid through the conduit I|5 to the fluid motor and thus advance the electrode |38 to the position shown in Figure 10. Closing of contacts C3 by-passes the thermocouple control of the high frequency current and the limit switch L52 so that high frequency current will be continuously supplied to the electrode |08 as long as it is in the position as shown in Figure 10. As previously referred to, this operation can be controlled by the position of the injection plunger |05 rather than by the pressure operated control if desired.

The injection plunger |05 continues its forward movement ejecting plastic material from the injection cylinder |00 until the cavities in the molds |34 and |35 are full. At this time the pressure for operating the hydraulic motor |01 increases due to the increased resistance met by the plunger |05 whereby a pressure operated control T2 is actuated to energize a contacter D for closing contacts D1 and D2 for opening contacts D3. Closing of contacts D2 energizes a timing control TB that regulates the dwell period of 'the injection plunger. Opening of contacts D3 de-energizes contactors C thereby opening contacts C2 and C3 to cle-energize solenoids ||9 to permit the hydraulic motor to reverse the position of the electrode |08 to the position shown in Figure 12 and simultaneously return the source of high frequency current under control of the thermo- .ber of the machine.

couple control TH to prevent overheating ofthe plastic mass in the injection cylinder and simultaneously stop the intensification of the heating effect in the end of the injection chamber |0|.

When the timing control TB has run its timed cycle, contacts TB1 are closed to energize contactor B and thereby close contacts B1 to condition the circuit ci' limit switch LS1 for operation and open contacts 1?3 to cle-energize the solenoid |24 to shift the valve |22 and permit fluid to be circulated through the conduit |2| to the motor |01 to return the injection plunger |05 to its retracted position whereupon limit switch LS1 is re-engaged and another cycle of operation can be performed.

In Figure 13 is schematically shown an injection machine of somewhat conventional type wherein the principle of high frequency electrostatic heating is used for heating the plastic material when it is fed through the injection cham- The conventional plastic injection machine works on a substantially continuous cycle wherein the injection plunger reciprocates within an injection cylinder for feeding charges of plastic material through the cylinder which are heated in their passage through the injection cylinder by heating means that conveys heat to the plastic material by surface conduction. In the arrangement shown in Figure 13 the injection machine is referred to as a somewhat conventional plastic injection machine bel cause the normal cycle of such machines is not interfered with, but the heating arrangement for the plastiomaterial has been altered to incorporate the use of high frequency electrostatic heating of the plastic material.

In this arrangement the injection machine consists of an injection cylinder |60 having an injection chamber |6| and a nozzle |62. Granular plastic material is fed into the injection chamber |6| by means of the feed screw |63 through a feed opening |64. Dies |65 and |66 are associated with the nozzle |62 to receive plastic material ejected through the nozzle from the injection chamber IGI.

An injection plunger |61 reciprocates within the injection chamber |6| and is actuated by means of a hydraulic motor |68 having a piston |69, the cylinder |60 receiving fluid through the fluid conducting conduits |10 and |1|.

A central electrode |12 is positioned axially within the injection plunger |61 and is electrically insulated from the same by means of the insulating sleeve |13. The electrode |12 is stationarily mounted upon the frame of the injection machine and as illustrated in the drawings, the electrode |12'is connected with an electrically insulating bushing |14 that is secured to a yoke |15 carried upon the hydraulic motor |68. It will thus be seen that the injection plunger |61 will reciprocate in the injection chamber |6| and upon the electrode |12.

The electrode has the front end |16 thereof spaced from the end wall |11 of the injection chamber |6| a distance equal to the radial distance between the periphery of the electrode |12 and the inner periphery of the injection chamber IBI. As previously described, when the electrode |12 is thus positioned within the injection chamber |6| a high frequency electrostatic field is set` up between the electrode |12 and the injection cylinder |60 so that the heating effect of the high frequency field upon the plastic material will be uniform throughout the entire mass of the material. It is, of course, understood that the electrode 12 is connected to one side of a source of high frequency current by means of an electric conductor |18 and the injection cylinder |60 is connected to the opposite side of the source of high frequency current by means of the electric conductor |19.

'I'he supply of high frequency current to the electrode |12 and the injection cylinder |60 can be regulated by means of a. temperature responsive control |80 that may be a thermo-couple control to prevent overheating of the plastic material Within the injection chamber |6I. As previously described with regard to the operation of the apparatus disclosed in Figure 8, the thermocouple control |80 can be short circuited at the time the injection plunger |61 is retracted beyond the feed opening |64 anduntil it is again advanced beyond the feed opening |64 to prevent heating of the plastic material when it is entering the injection chamber |6| due to operation of the feed screw |63. At all other times, the source of high frequency current can continuously supply current to the electrode |12 and the injection cylinder |60 to maintain a continuous high frequency field therebetween for heating the plastic material as it moves through the injection chamber |6|.

Any of the controls heretofore described can be used upon the injection machine, as shown in Figure 13, or normal'conventional controls can be used for regulating the'operation of the injection machine through what-may be termed as a normal injection cycle.

l As previously described a cooling chamber |8| `can be provided in the injection cylinder |60 to prevent overheating of the plastic material within the injection cylinder, or to prevent residual heat from effecting the temperature regulation of the plastic material passing through the injection cylinder.

vIn Figure 14 there is shown an extrusion apparatus for extruding plastic material wherein high frequency electrostatic heating is used for heating the plastic material in the extrusion chamber.

In this apparatus the machine consists of an extrusion cylinder |90 having an extrusion charnber |9| provided with a feed opening |92 through which plastic material is fed into the extrusion chamber |9|. The forward end |92 of the extrusion chamber |9| is provided with a tapering wall that converges toward an extrusion opening |93 provided in an extrusion die |94.

An extrusion screw |95 is positioned within the extrusion chamber |9| andA consists of a metal sleeve |96 upon which the flights |91 of the screw are mounted. An electrically insulating sleeve |98 is positioned Within the metal sleeve |96 and receives the shank |99 of an electrode 200 that has a tapered nose portion 20| contoured similar to the contour of the forward end of the extrusion chamber |92 so that the surface area of the nose 20| will be spaced equidistantly from the surface area of the forward end |92 of the extrusion chamber, whereby a high frequency field can be set up between the nose 20| of the extrusion screw and the extrusion cylinder |90 that will be uniformly acting throughout its area to uniformly heat plastic material moving therebetween in a manner as has heretofore been described. It is to be understood, of course, that the electrode 200 is connected to a source of high frequency current by means of the electric conductor 202 and the extrusion cylinder |90 is connected to the opposite terminal of the source of high frequency current by means of the electric conductor 203.

The extrusion screw can be driven by means of an electric motor 204 through a gear reduction mechanism 205 that is geared to the extrusion screw by means of gears 206.

Ii desired, the temperature of the plastic material within the forward end |92 of the extrusion chamber |9| can be controlled by means of a temperature responsive control member 201 such as a thermo-couple control in a manner heretofore described with regard to the other mechanisms previously referred to in this specication. Also, cooling chambers 208 and 209 can be provided in the extrusion cylinder 208 to more closely regulate the temperature of the plastic material passing through the extrusion chamber and to prevent residual heat from effecting the temperature regulation of the material. If desired, the chamber 209 can receive a heating fluid to preheat the plastic material in this portion of the extrusion machine s0 as to reduce somewhat the load carried by the high frequency electrostatic heating means. Also, the extrusion die |94 could have electrodes disposed therein to create an electrostatic field within the die for maintaining the desired temperature of the plastic material when passing through the die, this electrostatic field being under independent temperature control if desirable.

While the apparatus disclosed and described herein constitutes a preferred form of the invention, yet it is to be understood that the apparatus is capable of substantial alteration Without departing from the spirit of the invention, and that all such modifications as fall within the scope of the appended claims are intended to be included herein.

Having thus fully described my invention, what I claim as new and desire to secure by Letters Patent is:

1. The method of heating a body of plastic material by the use of high frequency current which consists of placing a body of plastic material between electrodes that have the surface areas thereof spaced substantially equidistantly throughout their lrespective areas, impressing a high-frequency electric current upon the electrodes to establish a field of high frequency energy therebetween to uniformly heat the body of plastic material while positioned therebetween, of subsequently moving one of the electrodes relative to the other to position a portion of their surface areas in substantially closer relationship without substantially aiecting the equidistantly spaced relationship of other portions of the areas thereof whereby to intensify the heating effect of the high frequency field between the areas of the electrodes that are spaced relatively close together, and moving the heated plastic material between the closely spaced areas of the electrodes to apply the intensified heating effect upon the material passing therebetween.A

2. A method of heating plastic material by the use of high frequency current which consists of positioning electrodes with their surface areas spaced equidistantly from one another, of applying a high frequency electric current upon the electrodes to establish a field of high frequency energy therebetween, of placing plastic material between the electrodes while their surfaces are maintained in equidistantly spaced relationship for uniformly heating the plastic material throughout its entire'mass by the application of high frequency current to the electrodes, of moving one of the electrodes relative to the other to position at least one portion of their surface areas in closer relationship than other portions thereof whereby the intensity of heating effect of the high frequency field between the portions of the electrodes that are in closer relationship is intensified, and of moving the plastic material that has been uniformly heated throughout its mass through the zone having' the intensified heating effect for increasing the temperature of the plastic material as it passes therethrough above the previous temperature level.

3. A method of heating plastic material by the use of high frequency energy which consists of placing plastic material between electrodes arranged to produce a zone having a uniformly acting high frequency field for uniformly heating the mass of plastic material throughout the entire body thereof while it is within said zone, of applying a high frequency electric current upon the electrodes to establish a field of high frequency energy therebetween, and of moving the heated plastic material between electrodes-arranged to produce a second zone having a high frequency field of greater intensity than of the first zone to thereby elevate the temperature of the heated plastic material as it passes through the second zone to above the temperature yobtained in the first zone.

4. A method of heating plasticmaterial in a plastic injection machine which consists, of creating, by the utilization of a high frequency electrical current, a high frequency field in the injection chamber of an injection machine that is uniformly acting throughout the neld, of placing plastic material within the uniformly acting high frequency field in the injection cylinder to uniformly heat the plastic material While within the field, of creating a second high frequency field within the injection cylinder of the machine adjacent to the nozzle thereof having a greater intensity than the first high frequency field, and of moving the plastic material heated in the first high frequency field through the second high frequency field to increase the temperature thereof 4 above the temperature created in the plastic material in the first high frequency field.

5. A method of heating plastic material by the use ofA high frequency energy which includes, producing, by the utilization of a high frequency electrical current, a zone of high frequency energy that is substantially uniformly acting throughout the zone, placing plastic material within the uniformly acting high frequency zone to raise the temperature thereof substantially uniformly throughout the mass of the plastic material, temporarily increasing the intensity of the high frequency in a portion of said zone above the intensity in the remainder thereof, moving the heated plastic material through the intensified portion of said zone while intensified to increase the temperature of the plastic material above the initial heating temperature while passing through the intensified zone, and subsequently reducing the intensity of the high frequency field in the said portion of said zone after a certain quantity of plastic material has passed therethrough whereby to retain a uniform heating effect upon the plastic material throughout the zone.

6. A method of heating plastic material by the use of high frequency energy which includes, producing, by the utilization of a high frequency electrical current, a zone of high frequency energy that is substantially uniformly acting throughout the zone, placing plastic material within the uniformly acting high frequency zone to raise the temperature thereof substantially terial above the initial heating temperature while passing through the intensified zone, and subsequently reducing the intensity of the high frequency field in the said portion of said zone substantially to the level of the initial intensity thereof after a certain quantity of plastic material has passed therethrough whereby to retain a uniform heating effect upon the plastic material throughout the zone.

'7. A method of heating plastic material that moves through the injection chamber of the plastic injection machine and is ejected through the nozzle thereof which includes, producing, by the utilization of a high frequency electrical current, a. zone of high frequency energy Within the injection chamber that is substantially uniformly acting .throughout the zone, moving plastic material into the uniformly acting zone of high frequency energy to uniformly heat the mass of plastic material while within the uniformly acting zone of high frequency energy, producing a second zone of high frequency energy within the Y injection chamber and adjacent to the nozzle thereof that has an intensity greater than the intensity of the first zone, and moving plastic material heated in the first zone through the second zone for raising the temperature thereof above the temperature produced in the first zone before entering the nozzle of the injection machine.

8. A method of heating plasticmaterial that moves through the injection chamber of the plastic injection machine and is ejected through the nozzle thereof which includes, producing, by the utilization of a high frequency electrical current, a zone of high frequency energy within the injection chamber that is substantially uniformly acting throughout the zone, moving plastic inaterial into the uniformly acting zone of high frequency energy to uniformly heat the mass of plastic material While within the uniformly acting zone of high frequency energy, producing a second zone of high frequency energy within the injection chamber and adjacent to the nozzle thereof that has an intensity greater than the intensity of the first zone, moving plastic material vheated in the first zone through the second zone for raising the temperature thereof above the temperature produced in the first zone before entering the nozzle of the injection machine, and reducing the intensity of the high frequency energy in the second zone to substantially the level of that in the first zone after a measured quantity of plastic material has passed through the second zone whereby to maintain a uniform heating effect upon the plastic material within the injection chamber.

9. A method of heating plastic material that moves through the injection chamber of the plastic injection machine and is ejected through the nozzle thereof which includes, producing, by the utilization of a high frequency electrical current, a zone of high frequency energy within the injection chamber that is substantially uniformly acting throughout the zone, moving plas- ,aseaoee tic material into the uniformly acting zone of high frequency energy to uniformly heat the mass of plastic material while within the uniformly acting zone` of high frequency energy, producing a second zone of high frequency energy within the injection chamber and adjacent to the nozzle thereof that has an intensity greater than the intensity of the first zone, moving pl stlc material heated in the first zone through he second zone for raising the temperature thereof above the temperature produced in the first zone before entering the nozzle of the injection machine, reducing the intensity of the high frequency energy in the second zone to substantially the level of that in the first zone after a measured quantity of plastic material has passed through the second zone whereby to maintain a uniform heating effect upon the plastic material within the injection chamber, and subsequently moving a small quantity of plastic material into the second zone from the first zone after the intensity of the high frequency current has been reduced in the second zone to place plastic material in the second zone that is substantially at the same temperature as the plastic material in the first zone.

10. A method of injection molding plastic materials in an injection machine having an injection cylinder provided with a nozzle that engages a mold and has an opening therein through which plastic material is moved into the mold and a plunger for moving the plastic material through theinjection cylinder which includes, feeding plastic material into the injection cylinder of the machine, producing, by the utilization of a high frequency electrical current, a zone of high frequency energy within the injection cylinder that is uniformly acting throughout the zone, moving the plastic material into the uniformly acting zone of high frequency to uniformly heat the plastic material while in the zone, temporarily increasing the intensity of the high frequency energy in a portion of said zone within the injection cylinder above the intensity in the remainder thereof and adjacent to the nozzle of the injection cylinder, moving the heated plastic material through the intensified portion of said zone while intensified to increase the temperature thereof above the temperature produced in the remaining portion of said zone before the material enters the nozzle of the machine, and reducing the intensity of the high frequency current in the intensified portion of said zone when the mold 4co-operating with the injection cylinder has been filled whereby to maintain a uniform heating effect upon the plastic material throughout the zone.

11. A method of injection molding plastic materials in an injection machine having an injection cylinder provided with a nozzle that engages a mold and has an opening therein through which plastic material is moved into the mold and a plunger for moving the plastic material through the injection cylinder which includes, feeding plastic material into the injection cylinder of the machine, producing, by the utilization of a high frequency electrical current, a zone of high frequency energy within the injection cylinder that is uniformly acting throughout the zone, i moving the plastic material into the uniformly acting zone of high frequency to uniformly heat the plastic material while in the zone, temporarily increasing the intensity of the high frequency energy in a portion of said zone Within the injection cylinder above the intensity in the remainder thereof and adjacent to the nozzle of the injection cylinder, moving the heated plastic material through the intensified portion of said zone while intensified to increase the temperature thereof above the temperature produced in the remaining portion of said zone before the material enters the nozzle of the machine, and reducing the intensity of the high frequency current in the intensified portion of said zone just prior to complete filling of the mold co-operating with the injection cylinder whereby a small quantity of plastic material passes through the said intensified zone for entry into the nozzle of the machine without having the temperature thereof elevated above the temperature of the plastic material in the unintensifled zone.

12. A method of heating plastic material within an injection machine having an injection cylinder and a nozzle therefor through which plastic material is moved by means of an injection plunger which includes, positioning an electrical conducting member Within the injection cylinder to conduct a high frequency current into the injection cylinder and establish a field of high frequency energy Within the injection cylinder that is uniformly acting throughout the area. of the injection cylinder, applying a high frequency electric current on the conducting member and the cylinder to produce a high frequency field therebetween, moving plastic material into the field of high frequency energy to uniformly raise the temperature of the plastic, material, moving the electrical conducting member relative to the injection cylinder to bring a portion thereof adjacent the nozzle for the injection cylinder and increase the intensity of the high frequency eld adjacent the nozzle of the injection cylinder, and moving the heated plastic material through the intensified high frequency field to further elevate the temperature 4 of the plastic material as it enters the nozzle for the injection cylinder.

13. A method of heating plastic material within an injection machine having an injection cylinder and a nozzle therefor through which plastic material is moved by means of an injection plunger which includes, positioning an electrical conducting member within the injection cylinder to conduct a high frequency current into the injection cylinder and establish a field of high frequency energy within the injection cylinder that is uniformly acting throughout the area of the injection cylinder, applying a high frequency electric current on the conducting member and the cylinder topioduce a high frequency field therebetween, moving plastic material into the eld of high frequency energy to uniformly raise the temperature of the plastic material, moving the electrical conducting member relative to the injection cylinder to bring a portion thereof adjacent the nozzle for the injection cylinder and increase the intensity of the high frequency field adjacent the nozzle of the injection cylinder, moving the heated plastic material through the intensified high frequency field to further elevate the temperature of the'plastic material as it enters the nozzle for the injection cylinder, and subsequently moving the electrical conducting member relative to the injection cylinder to return the same to initial position and thereby remove the intensified field of high frequency energy from adjacent the nozzle.

14. A method of heating plastic material Within an injection machine having an injection cylinder and a nozzle therefor through which plastic material is moved by means of an injection plunger which includes, positioning an electrical conducting member within the injection cylinder to conduct a high frequency current into the injection cylinder and establish a field of high frequency energy within the injection cylinder that is uniformly acting throughout the area of the injection cylinder, applying a high frequency electric current on the conducting member and the cylinder to produce a high frequency field therebetween, moving plastic material into the field of high frequency energy to uniformly raise the temperature of the plastic material, moving the electrical conducting member relative to the injection cylinder to bring a portion thereof adjacent the nozzle for the injection cylinder and increase the intensity of the high-frequency field adjacent the nozzle of the injection cylinder, moving the heated plastic material through the intensified high frequency field to further elevate the temperature of the plastic material as it enters the nozzle for the injection cylinder, and subsequently moving the electrical conducting member relative to the injection cylinder to return the same to initial position and thereby remove the intensified field of high frequency current from adjacent the nozzle prior to cornpletion of the injection stroke of the machine whereby plastic material as initially heated will be forced into the injection nozzle at completion of the injection stroke of the machine.

15. An injection molding apparatus having means for heating plastic material therein which includes, an injection cylinder, an injection plunger reciprocable in said cylinder, an electrode within said plunger extending from the end thereof to within said cylinder and spaced therefrom and over which said plunger reciprocates, `and a source of high frequency current connected to said cylinder and to said electrode for establishing a field of high frequency current therebetween.

16. An injection molding apparatus having means for heating plastic material therein which includes, an injection cylinder, an injection plunger reciprocable in said cylinder, an electrode within said plunger extending from the end thereof to within said injection cylinder in spaced relation thereto and to a position wherein the end of said electrode is spaced from the end of said injection cylinder a distance equal to the radial distance between the periphery of the electrode and the inner periphery of said cylinder, and a source of high frequency current connected to said injection cylinder and said electrode to establish a high frequency field therebetween having uniformly acting characteristics throughout the eld.

17. An injection molding apparatus having means for heating plastic material therein which includes, an injection cylinder, an injection plunger reciprocable in said cylinder, an electrode positioned within said cylinder and, adapted to extend from said plunger within said injection cylinder to a position wherein the and of said electrode is spaced from the end of said injection cylinder a distance equal to the radjal=- distance between the periphery of the electrode and the inner periphery of said cylinder, a source of high frequency current connected to said injection cylinder and said electrode to establish a high frequency field therebetween having uniformly acting characteristics throughout the field, and means for positioning the end of the electrode closely adjacent the end of the injecaseaeea tion cylinder to intensify the high frequency eld therebetween.

18. An injection machine for molding of plastic materials which includes, an injection cylinder, an injection plunger reciprocable in said cylinder, an electrode reciprocable within said plunger and adapted to extend from the end thereof into the injection cylinder, and independent means for reciprocating said injection plunger and said electrode relatively to one another.

19. An injection machine for injection molding of plastic material including, an injection cylinder, an injection plunger reciprocable therein, an electrode reciprocable within said plunger and electrically insulated from the same, said cylinder and said electrode being adapted to be connected to a source of high frequency, and means for reciprocating said electrode within said cylinder and within said plunger to extend the same beyond saidplunger, whereby a field of high frequency current can be established between said electrode and said cylinder for heating plastic material passing through said cylinder.

20. An injection machine for injection molding of plastic material including' an injection cylinder, an injection plunger reciprocable therein, an electrode reciprocable within said plunger and electrically insulated from the same, said cylinder and said electrode being adapted to be connected to a source of high frequency, electrode actuating means for positioning said electrode within said cylinder to extend the same beyond said plunger vwith the end of said electrode positioned from the end wall of said cylinder a distance substantiallyv equal to the radial distance between said cylinder and said electrode whereby a field of high frequency energy can be established therebetween having uniformly acting characteristics for uniformly heating plastic material disposed between said electrode and said cylinder, said electrode actuating means also providing means for positioning the end of said electrode within said cylinder closely adjacent the end wall of said cylinder whereby an intensied zone of high frequency energy may be obtained, and means for moving said plunger to move the heated plastic from the injection cylinder through the zone of intensified heating when ejecting the material from the cylinder.

2l. An injection machine for injection molding of plastic material including, an injection cylinder, an injection plunger reciprocable therein, an electrode reciprocable within said plunger and electrically insulated from the same, said cylinder and said electrode being adapted to be connected to a source vof high frequency, electrode actuating means for positioning said electrode within said cylinder to extend the same beyond said plunger with the end of said electrode positioned from the end wall of said cylinder a distance substantially equal to the radial distance between said cylinder and said electrode whereby a eld of high frequency energy can be established therebetween having uniformly acting'characteristics for uniformly heating plastic material disposed between said electrode and said cylinder, said electrode actuating means also providing means for positioning the end of said electrode within said cylinder closely adjacent the end wall of said cylinder whereby an intensified zone of high frequency energy may be 0btained, meanspfor moving said plunger to move the heated plastic from the injection cylinder trode within said cylinder to extend the same beyond said plunger with the end of said electrode positioned from the end Wall of said cylinder a distance substantially equal to the radial distance between said cylinder and said electrode whereby a field of high frequency energy can be established therebetween having uniformly acting characteristics for uniformly heating plastic material disposed between said electrode and said cylinder, said electrode actuating means also providing means for positioning the end of said electrode within said cylinder closely adjacent the end Wall of said cylinder whereby an intensifled zone of high frequency energy may be obtained, means for moving said plunger to move the heated plastic from the injection cylinder through the zone of intensified heating when ejecting the material from the cylinder, and means for withdrawing said electrode from its closely spaced position relative to the end wall of said cylinder prior to completion of the injection stroke of said plunger.

HOWARD' F. MACMILLIN. 

